ade7518 Analog Devices, Inc., ade7518 Datasheet - Page 47

• Manufacturer Part Number: ade7518
• Description: Single-phase Energy Measurement Ic With 8052 Mcu, Rtc, And Lcd Driver
• Manufacturer: Analog Devices, Inc.

Current Channel RMS Calculation
Each ADE7518 simultaneously calculates the rms values for the
current and voltage channels in different registers. Figure 48 shows
the detail of the signal processing chain for the rms calculation on
the current channel. The current channel rms value is processed
from the samples used in the current channel waveform sampling
mode and is stored in an unsigned 24-bit register (I
LSB of the current channel rms register is equivalent to one LSB
of a current channel waveform sample.
The update rate of the current channel rms measurement is
4.096 MHz/5. To minimize noise in the reading of the register,
the I
zero crossing of the voltage input. This configuration is done by
setting the ZXRMS bit in the MODE2 register (0x0C).
With the different specified full-scale analog input signal PGA1
values, the ADC produces an output code that is approximately
±0d2,147,483 (PGA1 = 1) or ±0d2,684,354 (PGA1 = 2, 4, 8, or 16).
See the Current Channel ADC section. Similarly, the equivalent
rms value of a full-scale ac signal is 0d1,518,499 (0x172BA3)
when PGA = 1 and 0d1,898,124 (0x1CF68C) when PGA1 = 2,
rms
register can also be configured to update only with the
I
P
HPF
Figure 48. Current Channel RMS Signal Processing with PGA1 = 1, 2, 4, 8, or 16
HPF1
0xD70A3E
0x28F5C2
0x000000
rms
24
). One
CURRENT CHANNEL
WAVEFORM
DATA RANGE
Rev. 0 | Page 47 of 128
LPF3
sgn
4, 8, or 16. The current rms measurement provided in the
ADE7518 is accurate to within 0.5% for signal inputs between
full scale and full scale/500. The conversion from the register
value to amps must be done externally in the microprocessor
using an amps/LSB constant.
Current Channel RMS Offset Compensation
The ADE7518 incorporates a current channel rms offset
compensation register (IRMSOS). This is a 12-bit signed register
that can be used to remove offset in the current channel rms
calculation. An offset can exist in the rms calculation due to
input noises that are integrated into the dc component of V
One LSB of the current channel rms offset is equivalent to
16,384 LSBs of the square of the current channel rms register.
Assuming that the maximum value from the current channel
rms calculation is 0d1,898,124 with full-scale ac inputs, then
1 LSB of the current channel rms offset represents 0.23% of
measurement error at −60 dB down from full scale.
where I
2
25
2
26
IRMSOS[11:0]
I
rms
2
rms0
27
+
=
is the rms measurement without offset correction.
2
18
I
rms
2
17
0
2
2
16
+
24
IRMSOS
0x00
I
rms
I
rms
×
(t)
[23:0]
32
,
768
ADE7518
2
(t).
(4)